Chronic fatigue syndrome (CFS) is a condition causing persistent fatigue that lasts for at least six months (or longer) and isn't a symptom of another medical condition (e.g., hypothyroidism). It was once thought to affect only highly educated young adults who are "high achievers" or career-oriented professionals. It is now known that CFS affects people of all ages and from all walks of life. CFS is about two to four times more common in women than in men. It is associated with extreme and prolonged fatigue that isn't relieved by rest. People with CFS experience persistent tiredness so severe that it prevents them from working, exercising, and enjoying life. CFS is a poorly understood condition and there is no clear consensus about its diagnosis and treatment.  CFS is also known as myalgic encephalomyelitis (ME) or chronic fatigue immune dysfunction syndrome (CFIDS), as it's called in the United States.